Process for stereoselective synthesis of lamivudine

ABSTRACT

A process for stereoselective synthesis of lamivudine, comprising (a) glycosylation of compounds of formula (I) and cytosine or protected cytosine, separating the resulting products by crystallization to afford intermediates of formula (II); (b) removing the protecting groups of intermediates of formula (II) to obtain lamivudine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Application No. PCT/CN2007/002038, filed on Jun. 29, 2007, which claims priority to Chinese Patent Application No. 200710040912.1, filed on May 18, 2007, the entire contents of both of which are incorporated herein by reference.

TECHNICAL FIELD

The present application relates to a process for stereoselective synthesis of Lamivudine.

BACKGROUND OF THE INVENTION

Lamivudine is a nucleoside reverse transcriptase inhibitor, and is a kind of deoxycytidine analogue, which can inhibit the reproduction of Human immunodeficiency virus (HIV) and hepatitis B virus (HBV), whose chemical name is (2R-cis)-4-amino-1-(2-hydroxymethyl-1,3-oxathiolan-5-yl)-1H-pyrimidin-2-one, and structural formula is as follows:

In 1990, Belleau et al firstly reported Lamivudine structure, and BioChem Pharma of Canada firstly developed Lamivudine to be used to treat AIDS (WO91/17159) and hepatitis B (EP0474119), and found that it had distinguished therapeutic effect on hepatitis B. Since Lamivudine has two chiral centers, it has 4 stereisomers, among which the 2R,5S (2R-cis)-isomer is the most potent in anti-HIV and anti-HBV activities, and its cytotoxicity on some cells is lower than its enatiomer or racemic body.

WO94/14802 mentioned two synthetic schemes (see Scheme 1 and Scheme 2):

In the above two schemes of this process, chirality was not controlled, and the final product was obtained by column chromatography, thus the yield was low and the requirement on the equipment was high, resulting in that the production cost was high and the operation in the production could not be controlled easily.

CN 1618795 has disclosed that the single isomer of cis-2-L-configuration is separated from the mixture which may comprise 2 to 4 stereoisomers by introducing a chiral auxiliary to the hydroxymethyl at position 2 and using fractional crystallization. The concrete scheme is as follows:

Although this patent has disclosed that a chiral auxiliary is introduced to the hydroxymethyl at position 2, it has not described that these chiral auxiliaries can enhance the stereoselectivity of glycosylated products. Therefore, it is believed that the function of introducing a chiral auxiliary is only to be used as a chiral resolution auxiliary. Accordingly, as to obtaining a desired single isomer, the yield is low and not suitable for industrial production.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a process for stereoselective synthesis of Lamivudine to overcome the deficiencies existing in the above techniques.

The technical concept of the present invention is as follows:

In order to obtain Lamivudine of high optical purity, it is assumed in the present invention that before the glycosylation reaction, the configuration of position 2 (R-configuration) of Lamivudine is fixed by using a chiral auxiliary as a chiral inducing agent, then the glycosylated products will only include a pair of diastereomers, which can be separated easily by the skilled in the art. It is proved by studies that this pair of diastereomers can be separated by recrystallization only.

The process for stereoselective synthesis of Lamivudine according to the present invention comprises the following steps:

(a) performing a glycosylation reaction between the compound of formula (I) and cytosine or protected cytosine, and separating the reaction product by recrystallization to obtain the intermediate of formula (II); and

(b) deprotecting the intermediate of formula (II) to obtain Lamivudine,

The specific reaction scheme is as follows:

wherein R₁ is a hydroxyl protecting group containing a chiral center, L is a leaving group; and R₂ is hydrogen or an amino protecting group.

In a preferred example of the present invention, preferably, R₁ is L-mentholformyl; L is methoxy or halogen or acetate group; and preferably, R₂ is hydrogen or acetyl.

In a preferred example of the present invention, step (a) can be specifically described as follows: cytosine or 4-amino protected cytosine is reacted with hexamethyldisilazane to give silylated 4-amino protected cytosine; which is then reacted with the compound of formula (I) at 10-80° C. for 1-20 hours; finally, the resulting glycosylated product is separated by recrystallization to give 2R,5S-intermediate of formula (II) of high optical purity.

The 4-amino protected cytosine is N⁴-acetylcytosine.

In another preferred example of the present invention, the recrystallization separation in step (a) is performed by a solvent.

The solvent is preferably an alcohol. The alcohol is preferably ethanol. Of course, the solvent is not limited to an alcohol, as long as it will not destroy the glycosylated product and can efficiently recrystallize and separate the diastereomers.

In a preferred example of the present invention, step (b) can be specifically described as follows: the intermediate of formula (II) from step (a) is hydrolyzed by a base, and forms a salt with an organic acid; then the salt precipitates from water and finally the water insoluble salt is neutralized by an organic base to give Lamivudine.

The organic acids include, but not limited to p-nitrobenzoic acid.

In a preferred example of the present invention, the synthetic route is preferably as follows:

In a preferred example of the present invention, the preparation method of the compound of formula (I) is as follows: the chiral auxiliary menthol is acylated by triphosgene to give L-menthol chloroformate, which is reacted with 1,2-isopropylidene protected glycerol, and then hydrolyzed to deprotect 1,2-isopropylidene; then the resulting product is oxidized by sodium periodate, and condensed with 1,4-dihydroxy-2,5-dithiane to give an intermediate of formula (VIII) of high optical purity; and, a hydroxyl at position 5 of the intermediate of formula (VIII) is substituted by chlorine to give a compound of formula (III), which belong to the compound of formula (I). The specific reaction scheme is as follows:

wherein X is Cl or Br.

In a preferred example of the present invention, the preparation method of the compound of formula (I) may also be as follows: a chiral compound of formula (IX) reported in WO95/29174 is used as a starting material, and methyl etherificated at 5-hydroxy, then reduced to give the intermediate of formula (X), then a chiral auxiliary is introduced to the hydroxy at position 2 of the intermediate of formula (X) to obtain the compound of formula (I). The specific reaction scheme is as follows:

This preparation method has a certain chiral control on the next glycosylation reaction.

The said method for introducing a chiral auxiliary comprises esterificating the acyl compound containing the chiral auxiliary with the intermediate of formula (X).

After studying the introduced chiral auxiliary, we have found that the preferred acyl compounds containing the chiral auxiliary include, but not limited to one of L-menthol formyl chloride, (S)-naproxenoyl chloride and (R)-methyl mandeloyl chloride. It has been found in the studies that some chiral groups have an inducing effect in the glycosylation reaction and afford the desired single isomer with high stereoselectivity. That is to say, the ratio of β isomer to α isomer in the product is larger than 1. While some chiral groups do not have an inducing effect and the ratio of the β isomer to α isomer is 1. If the group introduced to position 2 is not a chiral group, the ratio of the β isomer to α isomer is less than 1. Therefore, the acyl compound containing the chiral auxiliary is preferably L-menthol formyl chloride.

In order to enhance the yield of the glycosylation reaction, the intermediate of formula (IV) for chiral control can be transformed to a compound of formula (V), which then undergoes a glycosylation reaction with protected cytosine. In this manner, the expensive trifluoromethanesulfonic acid trimethylsilanol ester (a Lewis acid catalyst) can be replaced by trimethyliodosilane which is much cheaper, thus greatly reducing the cost of the starting materials. The reaction scheme is as follows:

By using the above process to produce Lamivudine, the product assay can reach above 99%, and the amount of its enantiomer is not high than 0.2%. The HPLC and chiral HPLC analytic methods can use the method as described in United States Pharmacopoeia USP29.

The present process has the following advantages: the reaction condition is mild, the stereoselectivity of the intermediate is high, the diastereomer can be separated by simple recrystallization, and the operation is simple and suitable for industrial production.

MODE FOR CARRYING OUT THE INVENTION

The present invention is further illustrated in detail by the Examples.

EXAMPLE 1 The preparation of L-menthol chloroformate

Under a nitrogen atmosphere, triphosgene (314.9 g, 1.19 mol) and toluene (6 L) were added to a reaction flask, and stirred to make triphosgene totally dissolved in toluene. At −10° C., L-menthol (546.5 g, 3.50 mol) was added, and stirred to make it dissolved in the above mixture, and then pyridine (451.0 g, 3.50 mol) was added dropwise. After the addition was complete, the reaction was conducted for 2 hours at that temperature. The reaction liquid was washed by water thrice (50 ml×3), the organic layer was dried by anhydrous sodium sulfate, and the solvent was evaporated under normal atmosphere to obtain oil, which was distilled under reduced pressure (5 mmHg). The fraction of 90-93° C. was collected to obtain the title compound, and the yield was 75%.

EXAMPLE 2 (2,2-dimethyl-1,3-dioxolan-4-yl)-methyl-(1R,2S,5R)-2-isopropyl-5-methyl cyclohexyl carbonic acid diester

At 0° C., L-menthol chloroformate (2.2 g, 0.01 mol), (2,2-dimethyl-1,3-dioxolan-4-yl)-methanol (2.2 g, 0.01 mol), N,N-dimethylamino pyridine (0.1 g, 1.0 mmol) and dichloromethane (15 ml) were added to a reaction flask. Triethylamine (3.2 g, 0.03 mol) was slowly added under vigorous stirring. After the addition was complete, the mixture was stirred for 0.5 hour, after which the reaction was stopped. The organic layer was washed by saturated sodium bicarbonate and water and then the solvent was evaporated. The crude product was purified by a silica gel column (eluted 

1. A process for stereoselective synthesis of Lamivudine, comprising the following steps: (a) performing a glycosylation reaction between the compound of formula (I) and cytosine or protected cytosine, and separating the reaction product by recrystallization to obtain the intermediate of formula (II); and (b) deprotecting the intermediate of formula (II) to obtain Lamivudine, wherein the reaction scheme is as follows:

wherein R₁ is a hydroxyl protecting group containing a chiral center, L is a leaving group; and R₂ is a hydrogen or an amino protecting group.
 2. The process for stereoselective synthesis of Lamivudine according to claim 1, wherein the preparation method of the compound of formula (I) is as follows: the chiral auxiliary menthol is acylated by triphosgene to give L-menthol chloroformate, which is reacted with 1,2-isopropylidene protected glycerol, and then hydrolyzed to deprotect 1,2-isopropylidene; the resulting product is oxidized by sodium periodate, and condensed with 1,4-dihydroxy-2,5-dithiane to give an intermediate of formula (VIII) of high optical purity; and a hydroxyl at position 5 of the intermediate of formula (VIII) is substituted by chlorine to give a compound of formula (III), which belongs to the compound of formula (I); the reaction scheme is as follows:

wherein X is Cl or Br.
 3. The process for stereoselective synthesis of Lamivudine according to claim 1, wherein the preparation method of the compound of formula (I) may also be as follows: a chiral compound of formula (IX) reported in WO95/29174 is used as a starting material, and methyl etherificated at 5-hydroxy, and then reduced to give the intermediate of formula (X), and then a chiral auxiliary is introduced to the hydroxy at position 2 of the intermediate of formula (X) to obtain the compound of formula (I); the reaction scheme is as follows:


4. The process for stereoselective synthesis of Lamivudine according to claim 3, wherein the said method for introducing a chiral auxiliary comprises esterificating the acyl compound containing the chiral auxiliary with the intermediate of formula (X).
 5. The process for stereoselective synthesis of Lamivudine according to claim 4, wherein the acyl compounds containing the chiral auxiliary is one of L-menthol formylchloride, (S)-naproxenoyl and (R)-methyl mandeloyl chloride.
 6. The process for stereoselective synthesis of Lamivudine according to claim 4, wherein the acyl compound containing the chiral auxiliary is L-menthol formylchloride.
 7. The process for stereoselective synthesis of Lamivudine according to claim 3, wherein the intermediate of formula (IV) for chiral control resulted form the compound of formula (X) is transformed to a compound of formula (V), which then undergoes a glycosylation reaction with protected cytosine, and the reaction scheme is as follows:
 8. The process for stereoselective synthesis of Lamivudine according to claim 1, wherein R₁ is L-mentholformyl.
 9. The process for stereoselective synthesis of Lamivudine according to claim 1, wherein R₂ is hydrogen or acetyl.
 10. The process for stereoselective synthesis of Lamivudine according to claim 1, wherein L is methoxy or halogen or acetate group.
 11. The process for stereoselective synthesis of Lamivudine according to claim 1, wherein step (a) can be described as follows: cytosine or 4-amino protected cytosine is reacted with hexamethyldisilazane to give silylated 4-amino protected cytosine; which is then reacted with the compound of formula (I) at 10-80° C. for 1-20 hours; finally the resulting glycosylated product is separated by recrystallization to give 2R,5S-intermediate of formula (II) of high optical purity.
 12. The process for stereoselective synthesis of Lamivudine according to claim 11, wherein the 4-amino protected cytosine is N⁴-acetylcytosine.
 13. The process for stereoselective synthesis of Lamivudine according to claim 1, 8, 9, 10 or 11, wherein step (b) can be described as follows: the intermediate of formula (II) from step (a) is hydrolyzed by a base, and forms a salt with an organic acid, then the salt precipitates from water and finally the water insoluble salt is neutralized by an organic base to give Lamivudine.
 14. The process for stereoselective synthesis of Lamivudine according to claim 13, wherein organic acid is p-nitrobenzoic acid.
 15. The process for stereoselective synthesis of Lamivudine according to claim 1, wherein the synthetic scheme is as follows: 